Electrode compositions based on an amorphous alloy having a high silicon content

ABSTRACT

An electrode composition for a lithium ion battery that includes an amorphous alloy having the formula Si x M y Al z  where x, y, and z represent atomic percent values and (a) x+y+z=100, (b) x≧55, (c) y&lt;22, (d) z&gt;0, and (e) M is one or more metals selected from the group consisting of manganese, molybdenum, niobium, tungsten, tantalum, iron, copper, titanium, vanadium, chromium, nickel, cobalt, zirconium, yttrium, and combinations thereof.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication No. 60/743,002, filed Dec. 1, 2005, the disclosure of whichis incorporated by reference herein in its entirety.

TECHNICAL FIELD

This invention relates to electrode compositions useful in lithium-ionbatteries.

BACKGROUND

Melt-spun alloys containing silicon, aluminum, and various transitionmetal elements have been proposed for use as electrodes for lithium-ionbatteries. These alloys have an amorphous microstructure that canenhance cycle life and thus overall battery performance. However, as thesilicon content increases, it becomes increasingly difficult to create acomposition having an amorphous microstructure. One proposed solution tothis problem requires using relatively high amounts (in terms of atomicpercent) and/or numbers of transition metal elements in combination withsilicon and aluminum. However, this proposed solution runs the risk ofrendering the materials electrochemically inactive.

SUMMARY

There is described an electrode composition for a lithium ion batterythat includes an amorphous alloy having the formula Si_(x)M_(y)Al_(z)where x, y, and z represent atomic percent values and (a) x+y+z=100, (b)x≧55, (c) y<22, (d) z>0, and (e) M is one or more metals selected fromthe group consisting of manganese, molybdenum, niobium, tungsten,tantalum, iron, copper, titanium, vanadium, chromium, nickel, cobalt,zirconium, yttrium, and combinations thereof. The amorphous alloy may bein the form of a single phase. An “amorphous alloy” is an alloy thatlacks long range atomic order and whose x-ray diffraction pattern lackssharp, well-defined peaks.

The value of x may be selected such that x≧60. The value of y may beselected such that y≦20.

M preferably represents no more than two metals. Specific examples ofparticularly useful metals include iron, titanium, zirconium, andcombinations thereof.

The electrode composition may be used as the anode for a lithium-ionbattery that also includes a cathode and an electrolyte. The electrolytemay include fluoroethylene carbonate. Preferably, the anode is in theform of a composite that includes the electrode composition incombination with a binder (e.g., a polyimide) and a conductive diluent(e.g., carbon black).

The electrode compositions exhibit high capacities and good cycle lifewhile at the same time minimizing the metal M content. The ability tominimize the metal M creates electrochemically active materials that areuseful as electrodes for lithium-ion batteries.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is an x-ray diffraction profile for the melt-spun alloy describedin Example 1.

FIG. 2 illustrates the cycling performance of an electrochemical cellincorporating the melt-spun alloy described in Example 1.

FIG. 3 is an x-ray diffraction profile for the melt-spun alloy describedin Example 2.

FIG. 4 illustrates the cycling performance of an electrochemical cellincorporating the melt-spun alloy described in Example 2.

DETAILED DESCRIPTION

Electrode compositions are described that are particularly useful asanodes for lithium-ion batteries. The electrode compositions feature anamorphous alloy having the formula Si_(x)M_(y)Al_(z) where x, y, and zrepresent atomic percent values and (a) x+y+z=100, (b) x≧55, (c) y<22,(d) z>0, and (e) M is one or more metals selected from the groupconsisting of manganese, molybdenum, niobium, tungsten, tantalum, iron,copper, titanium, vanadium, chromium, nickel, cobalt, zirconium,yttrium, and combinations thereof. Particularly useful compositions arethose in which x≧60, y≦20, and M represents no more than two metals.

The electrode compositions are preferably prepared by a chill block meltspinning process. Such processes are described generally, for example,in “Amorphous Metallic Alloys”, F. E. Luborsky, ed., Chapter 2,Butterworth & Co., Ltd. (London), 1983. According to this process,ingots containing silicon and the metal elements are melted in a radiofrequency field and then ejected through a nozzle onto the surface of arotating metal wheel (e.g., a copper or copper alloy wheel). Because thesurface temperature of the wheel is substantially lower than thetemperature of the melt, contact with the surface of the wheel quenchesthe melt. Quenching prevents the formation of large crystallites thatare detrimental to electrode performance. By using a wheel surface speedof greater than 40 m/s and an nozzle having a diameter less than 0.5 mm,amorphous compositions may be prepared.

The electrode compositions are particularly useful as anodes forlithium-ion batteries. The anode preferably is a composite in which theelectrode composition is combined with a binder and a conductivediluent. Examples of suitable binders include polyimides andpolyvinylidene fluoride. Examples of suitable conductive diluentsinclude carbon blacks.

To prepare a battery, the anode is combined with an electrolyte and acathode (the counterelectrode). The electrolyte may be in the form of aliquid, solid, or gel. Examples of solid electrolytes include polymericelectrolytes such as polyethylene oxide, fluorine-containing polymersand copolymers (e.g., polytetrafluoroethylene), and combinationsthereof. Examples of liquid electrolytes include ethylene carbonate,diethyl carbonate, propylene carbonate, fluoroethylene carbonate (FEC),and combinations thereof. The electrolyte is provided with a lithiumelectrolyte salt. Examples of suitable salts include LiPF₆, LiBF₄,lithium bis(oxalato)borate, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiAsF₆,LiC(CF₃SO₂)₃, and LiClO₄. Examples of suitable cathode compositionsinclude LiCoO₂, LiCo_(0.2)Ni_(0.8)O₂, and LiMn₂O₄. Additional examplesinclude the cathode compositions described in the following documents:(1) Lu et al., U.S. Publ. Pat. Appln. No. 2004/0121234; (2) Dahn et al.,U.S. Publ. Pat. Appln. No. 2003/0108793; (3) Eberman et al., U.S. Publ.Pat. Appln. No. 2005/0112054; (4) Dahn et al., U.S. Publ. Pat. Appln.No. 2004/0179993; (5) Lu et al., U.S. Pat. No. 6,964,828; (6) Lu et al.,U.S. Pat. No. 7,078,128; (7) Obrovac et al., U.S. Pat. No. 6,680,145;and (8) Dahn et al., U.S. Pat. No. 5,900,385.

EXAMPLES Example 1

13.93 g of aluminum, 49.960 g of silicon, 25.637 g of iron, and 10.469 gof zirconium (all 99.8% or better purity) were weighed in a weighingdish and then placed in an ARC furnace (available from Advanced VacuumSystems, Ayer, Mass.). The mixture was melted in an Ar atmosphere toyield an ingot having the composition Si₆₂Al₁₈Fe₁₆Zr₄, where all amountsare in atomic percent.

The ingot was cut into strips using a diamond blade wet saw to form 20 gstrips for melt spinning. The melt spinning apparatus consisted of avacuum chamber featuring a cylindrical quartz glass crucible. Thecrucible had a 16 mm internal diameter, a 140 mm length, and a 0.28 mmnozzle. The crucible was positioned above a rotating cooling wheel(Ni—Si—Cr—Cu C18000 Alloy containing 0.45 wt % Cr, 2.4 wt % Ni, 0.6 wt %Si, with the balance copper) having a thickness of 10 mm and a diameterof 203 mm. Prior to operation, the edge surface of the wheel waspolished using a rubbing compound (available as IMPERIAL MICROFINISHINGfrom 3M, St. Paul, Minn.) followed by a wipe with mineral oil to leave athin film.

For melt spinning, a 20 g ingot strip was placed in the crucible, afterwhich the system was evacuated to 80 mTorr and then filled with He gasto a pressure of 200 mTorr. The ingot was melted using RF induction. Asthe temperature reached 1275° C., a 400 mTorr He pressure was applied tothe surface of the molten alloy to extrude the alloy through the nozzleand onto the spinning wheel, which rotated at 5,031 rpm (53 m/s). Ribbonstrips having a width of 1 mm and a thickness of 10 microns wereproduced. The x-ray diffraction pattern of a representative strip wascollected using a Siemens Model Kristalloflex 805 D500 diffractometerequipped with a copper target x-ray tube and a diffracted beammonochromator. The results are shown in FIG. 1. The absence of sharppeaks was evidence of an amorphous composition.

1.70 g of the melt-spun ribbon, 150 mg of Super P carbon (a conductivediluent), 0.750 g of a polyimide coating solution (PYRALIN PI2555, 20%in N-methyl pyrollidone (NMP) available from HD Microsystems, ParlinkN.J.), and 3.75 g of NMP were added to a 40 ml tungsten carbide millingvessel containing a 10 mm diameter and a 10.3 mm diameter tungstencarbide ball. The vessel was placed in a planetary mill (PULVERISETTE 7,available from Fritsch GmbH, Idon-Oberstein, Germany) and milled at asetting of 8 for one hour.

Following milling, the solution was transferred to a notch coating barand coated onto a 15 micron thick Cu foil in a 25 mm wide, 125 micronthick strip. The coating was cured at 150° C. in vacuo for 2.5 hours toform the electrode. The electrode was then used to construct a 2225 coincell by combining it with a metallic lithium anode, two layers of a flatsheet polypropylene membrane separator (CELGARD 2400, available fromCelgard Inc., Charlotte, N.C.) and 1 M LiPF₆ in a 1:2 mixture ofethylene carbonate and diethyl carbonate as the electrolyte. The cellwas cycled using a battery cycler (MACCOR, Model 4000, available fromMaccor, Tulsa, Okla.) at a constant current of 0.125 mA between 0.9V and0.005V for the first cycle, and at a constant current of 0.5 mA between0.9V and 0.005V for all additional cycles. The results are shown in FIG.2. As shown in the figure, the cell exhibited good cycling performance.

Example 2

A melt-spun ingot was prepared following the procedure described inExample 1. The composition of the alloy was Si₅₅Al_(29.3)Fe_(15.7),where all amounts are in atomic percent. X-ray diffraction results,shown in FIG. 3, revealed a lack of sharp peaks, demonstrating that thecomposition was amorphous.

0.8 g of the melt-spun ribbon, 4.16 g of a dispersion of 3 wt. % Super Pcarbon (a conductive diluent), 3 wt. % polyvinylidene fluoride, 94%N-methyl pyrollidone (NMP), and 1 g of NMP were mixed together using ahigh-shear mixer for 15 min.

Following mixing, the slurry was transferred to a notch coating bar andcoated onto a 15 micron thick Cu foil in a 25 mm wide, 125 micron thickstrip. The coating was cured at 150° C. in vacuo for 2.5 hours to formthe electrode. The electrode was then used to construct a 2225 coin cellby combining it with a metallic lithium anode, two layers of CELGARD2400 as the separator, and 1 M LiPF₆ in a 1:2 mixture of ethylenecarbonate and diethyl carbonate as the electrolyte. The cell was cycledusing a MACCOR cycler according to the protocol described in Table 1.The results are shown in FIG. 4. As shown in the figure, the cellexhibited good cycling performance.

TABLE 1 Cycle Current Voltage Trickle lithiation #1 70 mA/g 5 mV  7 mA/gdelithiation #1 70 mA/g 0.9 V  7 mA/g lithiation #2 70 mA/g 5 mV 14 mA/gdelithiation #2 70 mA/g 0.9 V none lithiation #3+ 140 mA/g  5 mV 14 mA/gdelithiation #3+ 140 mA/g  0.9 V none

Examples 3 and 4

Samples having the composition set forth in Table 2 were preparedaccording to the procedure described in Example 1. X-ray diffractiondata revealed a lack of sharp peaks in both cases, demonstrating thatthe compositions were amorphous. The cycling capacity of electrochemicalcells prepared using each composition are also set forth in Table 2. Theresults demonstrate that each cell exhibited good cycling behavior.

TABLE 2 Composition (atomic %) Example Si Al Fe Ti Cycling Capacity 3 6020 12 8 1200 mAh/g 4 62 16 14 8  700 mAh/g

A number of embodiments of the invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.Accordingly, other embodiments are within the scope of the followingclaims.

1. An electrode composition for a lithium ion battery comprising anamorphous alloy having the formula Si_(x)M_(y)Al_(z) where x, y, and zrepresent atomic percent values and: (a) x+y+z=100; (b) x≧55; (c)15<y<22; (d) z>0; and (e) M represents no more than two metals selectedfrom the group consisting of manganese, molybdenum, niobium, tungsten,tantalum, iron, copper, titanium, vanadium, chromium, nickel, cobalt,zirconium, yttrium, and combinations thereof, and M includes titanium,zirconium, or both.
 2. An electrode composition according to claim 1wherein x≧60.
 3. An electrode composition according to claim 1 whereiny≦20.
 4. An electrode composition according to claim 1 wherein x≧60 andy≦20.
 5. An electrode composition according to claim 1 wherein theamorphous alloy is in the form of a single phase.
 6. A lithium ionbattery comprising: (a) an anode; (b) a cathode; and (c) an electrolyte,wherein the anode comprises an amorphous alloy having the formulaSi_(x)M_(y)Al_(z) where x, y, and z represent atomic percent values and:(i) x+y+z=100; (ii) x≧55; (iii) 15<y<22; (iv) z>0; and (v) M representsno more than two metals selected from the group consisting of manganese,molybdenum, niobium, tungsten, tantalum, iron, copper, titanium,vanadium, chromium, nickel, cobalt, zirconium, yttrium, and combinationsthereof, and M includes titanium, zirconium, or both.
 7. A lithium ionbattery according to claim 6 wherein x≧60.
 8. A lithium ion batteryaccording to claim 6 wherein y≦20.
 9. A lithium ion battery according toclaim 6 wherein x≧60 and y≦20.
 10. A lithium ion battery according toclaim 6 wherein the amorphous alloy is in the form of a single phase.11. A lithium ion battery according to claim 6 wherein the anode furthercomprises a binder and a conductive diluent.
 12. A lithium ion batteryaccording to claim 11 wherein the binder comprises a polyimide.
 13. Alithium ion battery according to claim 11 wherein the conductive diluentcomprises carbon black.
 14. A lithium ion battery according to claim 6wherein the electrolyte comprises fluoroethylene carbonate.